Chromate coating composition for aluminum and process



United States Patent 3,380,858 CHROMATE COATING COMPOSITION FOR ALUMINUMAND PROCESS Ashok J. Champaueria, Detroit, and William S. Russell,

Warren, Mich, assignors to Hooker Chemical Corporation, Niagara Falls,N.Y., a corporation of New York No Drawing. Filed Nov. 10, 1964, Ser.No. 410,286

16 Claims. (Cl. 148-62) ABSTRACT OF THE DISCLOSURE A compositionsuitable for forming a protective coating on aluminum containingsurfaces which consists essentially of an aqueous acidic solutioncontaining hexavalent chromium ions, fluoride ions, and 0.01 to 0.4% byweight of uranium, as an activator for the composition. Thealuminum-containing surfaces are brought in contact with this solution,using any suitable contacting technique, for a period sufiicient to formthe desired protective coating on the aluminum-containing surface.

This invention relates to an improved composition and process forcoating metal surfaces and more particularly relates to improvecompositions and methods for chemically coating aluminum and aluminumalloy surfaces'to provide corrosion resistant and paint bonding coatingon such surfaces.

Many compositions and processes have been proposed in the past forforming chemical films on surfaces of aluminum, and particularlysurfaces of aluminum and aluminum alloys. From a commercial standpoint,the most successful of these processes are those wherein the aluminumcontaining surface to be coated is contacted with an acidic solutioncontaining a source of hexavalent chromium. In such, the attempt hasbeen made to provide method which is not only useful in treating varioustypes of aluminum and aluminum alloys, but also to provide a coatingsolution which is realtively simple to prepare initially and which iseasily maintained in effective coating condition during use.

For the most part, these prior 'art compositions have been aqueousacidic solutions containing a source of hexavalent chromium ions and asource of fluoride ions. It has also been the general practice to add tothese basic compositions various additional anions or cations in orderto obtain specific objectives, such as activation of the solution inorder to obtain increased coating weight and coating efficiency.Exemplary of such modified compositions is that set forth in US. Patent2,796,370, wherein ferricyanide is utilized as the modifying oractivating anion.

In the commercial operation of chromate-fiuoride ferricyanide activatedsolutions for coating aluminum, a number of operational difiicultieshave been encountered. These solutions must always contain a certainminimum quantity of ferricyanide ion in order to preserve the coatingrate and coating efficiency in the solution at levels which arecommercially acceptable from the standpoints of both quality andeconomy. It is known that the ferricyanide ion is sensitive totemperature degradation at temperatures above about 50 degrees C., andunless care is constantly exercised to avoid exceeding this temperature,even at localized areas within the operating tank, the ferricyanidebecomes degraded and incapable of maintaining the solution in acceptablecoating-forming condition. Additionally, it has been found that theseferricyanide activated solutions are relatively sensitive to acidityand, thus, require considerable maintenance of the necessary aciditywithin a fairly narrow range. Moreover, these solutions are found tolose their coating ability merely upon standing at room temperaturewithout use and the replenishment of them is complicated because theferricyanide and chromate-fiuoride component must be added separately tothe solution and must be maintained, prior to addition, in separatecontainers in order to avoid reaction between these components.

It is therefore, an object of the present invention to provide animproved chromate-fluoride coating solution which is capable ofoperations at high speed and high coating efficiencies to form acorrosion resistant, paint receptive coating on aluminum containingsurfaces.

Another object of the present invention is to provide an improvedchromate-fiuoride coating solution which is capable of operating at highcoating rates and high coating efliciencies over a wide range of pH andtemperature values.

A further object of the present invention is to provide an improvedprocess for coating aluminum containing surfaces, which process iscapable of being maintained in optimum coating-forming condition byreplenishing with a single replenishing material.

A still further object of the invention is to provide an improvedprocess for coating aluminum surfaces which process is economical touse, easy to control and maintain in optimum coating-forming conditionand which is capable of providing adherent, corrosion resistantcoatings.

These and other objects of the invention will become apparent to thoseskilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a coatingcomposition which comprises an aqueous acidic solution, substantiallyfree of sulfate ions and which contains hexavalent chromium ions,fluoride ions and 0.01 to 0.4 percent of uranium.

More specifically, the composition of the present invention is anaqueous acidic solution which is substantially free of sulfate ions,i.e., 805*. It has been found, in the present composition, that not onlydo these ions not add any beneficial properties to the composition but,in many instances, are in fact detrimental to the operation andmaintenance of the coating solution and to the protective coatings whichare produced. Accordingly, it is desirable that the coating solutions ofthe present invention be substantially free of these ions. Bysubstantially free, it is meant that the solutions are free of at leastamounts of these ions which are detrimental to the solution or coatingproduced. It is not necessarily intended, however, to exclude minoramounts of these ions which are not detrimental, such as amounts Whichmay occur in the water used in making up the aqueous solution, e.g.,amounts not greater than about 0.05% by weight of the solution.

The aqueous acidic coating solutions of the present invention containhexavalent chromium ions in an amount sufiicient to provide the desiredchromium coating on the aluminum. surfaces treated therewith. Desirably,these solutions contain hexavalent chromium ions, calculated as CrO inan amount within the range of about 0.05 to 1 percent, weight/volume(w./v.), i.e., percent weight per unit volume. The hexavalent chromiumions may be added to the solution in many suitable forms, such aschromic acid, or one or more of the water soluble or water dispersablesalts of chromic acid. Exemplary of the salts which may be used are thesodium, potassium or ammonium salts such as the chromates anddichromates, as well as admixtures thereof both with each other and/orwith chromic acid.

The fluoride ion is present in the subject composition in an amountsufficient to cause attack of the aluminum containing surface to becoated and to effect formation of the resulting coating on this surface.Desirably, the fluoride ion is present in an amount within the range ofabout 0.16 to about 2.7 percent w./v. As with the hexavalent chromiumion, the fluoride ions may be added to the present composition in manyconvenient forms, including various fluorine-containing compounds whichare capable of ionizing in the aqueous acidic solutions of the inventionto provide fluoride ions. Exemplary of such fluorine containingcompounds which may be used are hydrofluoric acid, fluosilic acid,fluoboric acid, as well as the various water soluble or waterdispersable salts thereof such as the sodium, potassium, and ammoniumsalts.

As has been indicated hereinabove, there is also included in the presentcomposition, 0.01 to 0.4 percent w./v. of uranium. It is believed thatin the present composition, the uranium acts as an activator which makesit possible to obtain and maintain high coating weights and coatingefiiciencies. The uranium may be added to the coating solution in theform of various compounds which are ionizable in the solution, such asuranic acid, as well as various water soluble or water dispersable saltsthereof which will provide the desired uranium ion when oxidized in thesolution by the chromic acid or chromic acid salts. Exemplary of suchuranium containing salts which may be used are the sodium, potassium, orammonium salts or uranic acid.

In addition to the above components, in many instances it has also beenfound desirable to include in the present composition an aluminumfluoride complex ion, which ion is desirably present in an amountequivalent to about 0.22 to about 3.2 percent W./v. of Al(F) Thealuminum fluoride complex ion is expressed as Al(F) because, when used,it is present in the operating solution as an equilibrium of Al(F) ionswhich may contain from 1 to 6 fluoride atoms per aluminum atom. In thesolutions of the present invention, it has been found that thisequilibrium averages out to be approximately equivalent to the Al(F)ion. Accordingly, as used in the specification and claims, theexpression Al(F) is intended to represent any aluminum fluoride ion andthe quantities thereof refer to an amount of such ion equivalent to theAl(F),-, ion. This aluminum fluoride complex ion, when used, may beincorporated in the present coating solution as such, or may be formedin the solution as a complex from free aluminum and fluoride ions. Inthe latter instance, the fluoride may be present as hydrofluoric acid,fluoboric acid, fluosilicic acid, or the like. Where the aluminumfluoride complex is added as such, it may be prepared by dissolvingaluminum oxide (A1 in water and hydrofluoric acid in appropriateproportions to obtain the requisite parts of Al(F) for the composition.

Particularly preferred compositions of the present invention are thosefalling within the following formulation- Concentration, Solutioncomponent: percent w./v.

Hexavalent chromium ions (calculated The solutions of the presentinvention may be used to form coatings on surfaces containing aluminum,such as aluminum itself or aluminum alloys which are predominantlyaluminum, using various coating techniques, such as dipping, brushing,spraying, flooding, or the like. Preferably, the solutions of thepresent invention are applied to the aluminum surfaces after thesesurfaces have been subjected to conventional cleaning procedures whichfree the surface to be treated of oil, grease, oxides, and the like.Additionally, the present solutions may be applied to the aluminumsurfaces by atomizing the solution on the surface in a heated condition.Generally stated, this atomization application technique includes thesteps of preliminarily heating the aluminum or aluminum alloy surface tobe coated to a temperature above about C. and atomizing on the heatedsurface a quantity of the coating solution sufficient to form thedesired coating but insuflicient to cause the droplets of atomizedcoating solution to coalesce or puddle on the surface. The coating onthe aluminum surface results from the substantially instantaneousflashing or volitilization of the liquid from the solution, so that eachindividual atomized particle droplet remains substantially in the locusof its original contact with the surface treated.

Although the coating process of the present invention may be carried outeffectively and with good efficiency over a wide range of solutiontemperatures and solution acidities, it has been found that the rate ofcoating may be improved and the coating efficiency increased by theconcurrent selection and control of the degree of the acidity of thesolution and its temperature of application. With regard to thetemperature, it has been found that as the temperature of the operatingsolution is increased from room temperature, i.e., about 20? C., up toabout 50 C., the coating rate rapidly increases and in some instances,it is possible to obtain an increase in the coating rate of from 2 to 5times that obtained at room temperature. At solution temperatures withinthe range of about 50 to about C., the coating rate has been found toincrease much more slowly and, for all practical purposes, has beenfound to be substantially uniform throughout this temperature range. Itis, therefore, preferred to utilize the solutions of the presentinvention at solution temperatures within the range of about 50 to about70 C. Higher temperatures than 70 C. may be employed, for exampletemperatures of C., or even up to the solution boiling point, but noparticular advantages in terms of increased coating rates, are obtainedby operating at such higher temperatures.

With regard to the pH of the operating solutions of the presentinvention, it has been found that this, as with the temperature, affectsthe coating rate andgcoating efliciencies of the solution being appliedto the aluminum containing surfaces. Accordingly, it is desirable thatthe coating solution have a pH within the range of about 1.1 to about2.3, and preferablyin the range of about 1.6 to about 2.1. This pH rangerefers to measurements taken by using an electrical pH meter, employinga glass electrode and a calomel electrode, by emersing the electrodes infresh portions of the operating solution and observing the indicatedvalues.

In addition to the pH of the operating solution, it is also desirablethat the operating solution have a concentration within the range ofabout 7 to about 15 points and that once the concentration isestablished within this range it is maintained within about $0.5 pointsof the established value. The concentration of the operating solution inpoints, isdetermined by the following procedure:

To a ten millimeter sample of the operating solution there is added 25millimeters of 50 percent sulfuric acid and 2 drops oforthophenanthroline ferrous complex (ferroin) indicator. This solutionis then titrated with 0.1 N ferrous sulphate in dilute sulphuric aciduntil the solution changes through blue to a reddish-brown color. Theconcentration points of the operating solution are the number ofmillimeters of the 0.1 N titrating solution used. It is to beappreciated, that although the operating solution of the presentinvention is desirably used at a concentration within the range of about7 to 15 points, operation of the solution at both higher and lower pointconcentration is not only possible, but in some instances, is preferred.

Following the application of the coating solution of the presentinvention to the aluminum containing surfaces to be treated, thethus-coated surfaces are then desirably rinsed with water. Either sprayor emersion techniques for the water rinse may be used, with rinsingtimes of about 3 to 5 seconds duration being typical. Following thewater rinse, if desired, the coated surface may be given an additionalrinse with deionized water or with a dilute solution of hexavalentchromium, e.g. CrO This latter rinse is preferably effected 'byspraying, rinsing times of about 3 to 5 seconds duration at temperatureswithin the range of about 55 to 65 C., being typical. After the rinsingof the coated aluminum surface has been completed, the surfaces arepreferably dried so as to remove any surface moisture. The coatingsthus-produced on the aluminum surfaces are slightly colored and vary inappearance from iridescent to light gold to yellow to brown. The colorchanges in the coatings produced may be used as a guide to the coatingweights obtained, the darker colors being produced with higher coatingweights and the lighter colors resulting from lower coating weights.

In formulating the operating solutions of the present invention, amake-up composition, containing the components desired in the operatingsolution, is admixed with water in amounts suitable to provideconcentrations of the components within the ranges as set forthhereinabove. Normally, in addition to the above indicated components,the make-up composition may also contain an inorganic acid such asnitric acid or the like, in order to provide the desired acidity or pH.Suitable make-up compositions are those falling within the followingformulation Component: Parts by weight CrO 15-2O HF 4-7 Inorganic acid,such as HNO 1-5 Uranium activator composition 2-5 Al(F) (average AiFg)36 It will be appreciated that this is a single package make-upcomposition as compared to the prior art compositions wherein separatepackaging of some of the make-up components was often necessary.

In the operation of the process of the present invention, the componentsof the coating solution are depleted. Accordingly, in order to maintainthese components in the operating solution within the preferred rangeswhich have been set forth hereinabove, it is desirable, in order toobtain a continuous operation, to periodically replenish the operatingsolution. One advantage of the present composition is that as informulating the operating solution, this replenishing may be effectedusing a single package replenishing material, as opposed to many of theprior art compositions wherein separate addition of the componentsduring replenishing is necessary. As with the make-up composition, inaddition to the hexavalent chro mium, fluoride, uranium and aluminumfluoride complex components, it may also be desirable to include in thereplenishing material an inorganic acid, such as nitric acid,hydrochloric acid, and the like, so as to maintain the acidity or pH ofthe operating solution within the desired ranges as have been indicatedherein above. Addi tionally, where the fluoride component is added ashydrogen fluoride, rather than fluosilic acid or fluoboric acid, it mayalso be desirable to include in the replenishing material a quantity ofboric acid, to act as a buffer for the fluoride ions. This addition ofboric acid may also be desirable in making up the original operatingsolution, when the fluoride ion is added as hydrogen fluoride, andparticularly where the aluminum fluoride complex ion is not included inthe original solution. Where boric acid is included in the originaloperating solution, it is typically Components: Parts by weightHexavalent chromium (calculated as CrO 15 to 20 F 7 to 12 Acid 1.4 to 7Alkali metal uranate 3 to 6 5 AMP) (average Al(F) 0.6 to 1.6

Additionally, this composition may also contain about 1.2 to about 2parts by weight of boric acid (ll-1 A preferred replenishing material,having particular uitility for use with a continuous strip lineoperation, is one containing the following components in the amountsh1dicated Components: Parts by weight CrO 18 to 20 HF 10 to 12 HNO 5 to7 Sodium uranate 3 to 6 Al(F) X (average A1F 0.8 to 1.5

A preferred replenishing material, particularly suitable for productionoperations in which the parts to be coated are moved through a sprayinstallation on a monorail conveyor, is one having the followingcomponents in the amounts indicated Components: Parts by weight CrO 15to 17 HF 8 to 11 HNO 4.5 to 6.5 Sodium uranate 3.5 to 6.5 Al(F) (averageAlF 1 to 1.6

As has been indicated hereinabove, the presence in the operatingsolution of the present invention of foreign cations, that is, cationsother than the sodium, potassium or ammonium ions normally introduced,together with the fluoride or uranium ions, have been found to beundesirable and to detrimentally affect the coating rate and coatingefliciencies which may be obtained by using the solution of the presentinvention in the manner in which it has been described above. Inparticular, trivalent chromium ions in amounts in excess of about 0.1percent by weight of the solution have been found to have an appreciablyadverse affect on the coating rate and coating efficiencies obtained.Accordingly, it has been found to be particularly advantageous tomaintain the solutions of the present invention in optimum operatingconditions by operating these solutions in conjunction with an ionexchange unit of the type and by the procedures described in US. Patent2,967,791, issued Jan. 10, 1961. In operating the present solutions withsuch an ion exchange unit, which employs a cation exchange resin, thesolutions are maintained free of metallic cations other than aluminum,which occur in the solutions as aluminum fluoride complex ions, andthese complexes are controlled in concentration in the solution by theion exchange unit.

By the method of the present invention, corrosion resistant, adherentcoatings are formed on the aluminum or aluminum alloy surfaces treatedin the matter of a few seconds, typically about 1 to about 20 seconds.Moreover, by modifying the composition as to the uranium content,fluoride content, aluminum fluoride complex content, as well as to pH inacidity, it is possible to maintain a coating rate which is sufiicientlyfast as to form coatings on continuous strips of aluminum or aluminumalloys which coatings have a weight in the range of about 15 to about 40milligrams per square foot, in contact times of about 1 to 2 seconds.

In order that those skilled in the art may better understand the'present invention and the manner in which it may be practiced, thefollowing specific examples are given. It is to be understood, thatthese examples are presented for illustrative purposes only and are notintended to be taken as a limitation on the present invention. In theseexamples, unless otherwise indicated, temperatures are in degreescentigrade and amounts are in percent weight/volume. Additionally, theterm Coating Efficiency refers to the quantity of coating formedrelative to the amount of metal dissolved from the surface of the metalbeing coated and is, specifically, the ratio obtained by dividing themetal loss, in milligrams per square foot of surface treated, by thecoating weight on the same area, in milligrams per square foot. Thus, asthis numerical ratio decreases, the coating efliciency increases and thelowest numbers represent the h hest efliciency of coating formation.

Example 1 An aqueous coating solution was formulated containing thefollowing components in the amounts indicated:

Components: Percent, w./v. CF03 F- 0.70 Al+ 0 30 Cr 003 H BO 020 Thissolution, which had a pH of 1.4, was heated to 55 C. and panels of 3003aluminum where coated by spraying with the solution for 15 seconds. Thecoating weight obtained was 44.4 milligrams per square foot and thecoating efliciency was 0.59. Thereafter, additions of sodium uranatewere made to the solution and after each addition panels were coated asabove with the following results:

Additives of Sodium Coating Weights in Uranate m Grams Milligrams PerSquare Coating Elficiencics Foot 1 47v 1 0. 403 1 43. 1 0. 395 1 49. 2O. 376 1 51 6 O. 400 1 50. 4 0. 376

Example 2 A solution was prepared containing 0.25% CrO 0.02% fluoride,as HF, 0.02% uranium, added as sodium uranate, and 0.22% Al(F) (averageMB). The solution had a pH of 1.68, a total fluoride of 0.17% and whenapplied by spraying at 120 F. for 7-15 seconds contact time to aluminumbuilder stock 3105, produced coatings that had a coating weight of about50 milligrams per square foot and a coating efliciency of 0.25.

The above examples are repeated using other sources of hexavalentchromium and fluoride, including sodium and potassium dechromate andfluosilic acid, respectively to obtain similar results. Thethus-produced coatings are found to be excellent paint base coatingsand, when painted, give very good results when subjected to the 5% saltspray, humidity, adhesion and other physical tests.

While there have been described various embodiments of the invention,the compositions and methods described are not intended to be understoodas limiting the scope of the invention as it is realized that changestherewithin are possible and it is further intended that each elementrecited in any of the following claims is intended to be understood asreferring to all equivalent elements for accomplishing substantially thesame results in substantially the same or equivalent manner, it beingintended to cover the invention broadly in whatever form its principlesmay be utilized.

What is claimed is:

1. A composition suitable for forming a protective coating on aluminumcontaining surfaces which consists essentially of an aqueous acidicsolution which contains an effective coating amount of hexavalentchromium ions, fluoride ions in an amount effective to attack thealuminum suface being treated and 0.01 to 0.4 percent of uranium.

2. The composition as claimed as in claim 1 wherein there is alsopresent an aluminum fluoride complex.

3. A composition suitable for forming a protective coating on aluminumcontaining surfaces which consists essentially of an aqueous acidicsolution which contains, in percent w./v., 0.05 to 1 hexavalent chromiumion, calculated as CrO 0.16 to 2.7 fluoride ions, and 0.01 to 0.4uranium.

4. The composition as claimed in claim 3 wherein the solution alsocontains, in percent w./v., 0.22 to 3.2 Al(F) complex.

5. The composition as claimed in claim 4 wherein the solution containsin percent w./v., 0.2 to 0.5 hexavalent chromium ion, calculated as CrO0.16 to 1.6 fluoride ions, 0.03 to 0.1 uranium, and 0.22 to 1.9 Al(F)complex.

6. A method of forming a protective coating of aluminum-containingsurfaces which comprises contacting the surface to be treated with anaqueous acidic solution which consists essentially of an effectivecoating amount of hexavalent chromium ions, fluoride ions in an amounteffective to attack the surface being treated, and 0.01% to 0.4% ofuranium and maintaining the said aqueous acidic solution in contact withthe surface to be treated until the desired protective coating isformed.

7. The method as claimed in claim 6 wherein the aqueous acidic solutionalso contains an aluminum fluoride complex ion.

8. A method for forming a protective coating on aluminum-containingsurfaces which comprises contacting the surface to be treated with anaqueous acidic solution which consists essentially of in percent w./v.,0.05 to 1 hexavalent chromium ion calculated as CrO 0.1 to 2.7 fluorideions, and 0.01 to 0.4 uranium, and maintaining the said aqueous acidicsolution in contact with the surface to be treated until the desiredprotective coating is formed.

9. The method is as claimed in claim 8 wherein the aqueous acidicsolution also contains, in percent w./v., 0.22 to 3.2 Al(F) complex.

10. The method as claimed in claim 9 wherein the aqueous acidic solutioncontains, in percent w./v., 0.2 to 0.5 hexavalent chromium ion,calculated as CrO 0.16 to 1.6 fluoride ions, 0.03 to 0.1 uranium, and0.22 to 1.9 Al(F) complex.

11. A replenishing material for forming solutions for coating aluminumcontaining surfaces which consists essentially of, in parts by weight,15 to 20 CrO 7 to 12 HF; 1.4 to 7 inorganic acid; 3 to 6.5 alkali metaluranate; and 0.6 to 1.6 Al(F) (average A11 12. The replenishing materialas claimed in claim 11 wherein there is contained in parts by weight, 18to 20 CrO 10 to 12 HP; 5 to 7 HNO 3 to 6 sodium uranate; and 0.8 to 1.5Al(F) 13. The replenishing composition as claimed in claim 11 whereinthere is contained in parts by Weight, 15 to 17 CrO 8 to 11 HF; 4.5 to6.5 HNO 3.5 to 6.5 sodium uranate; and 1 to 1.6 Al(F) 14. An aluminumsurface having a coating thereon produced in accordance with the methodas claimed in claim 6.

r r 0,38%,388 9 i9 15. An aluminum surface having a coating thereonReferences Cited produced accordance with the method of claim 19. UNITEDSTATES PATENTS 16. A make-up composition, suitable for forming aqueousacid solutions for coating aluminum-containing sur- 1,811,298 6/1931Boulanger 143-6-27 X faces which consists essentially of, in parts byweight, 15 5 2,851,385 9/1958 Spurallce et 148-537 X to 20 CZO3; 4 to 7HP; 1 to 5 inorganic acid; 2 to 5 3,113,051 12/1963 Pimbley et a1 148-62uranium activator composition; and 3 to 6 AMP) (aver- 3 329 536 7 19 7Lcdeesen et 1 143 27 X age A11 wherein the activator composition isformed of a water soluble salt of uranic acid. RALPH S. KENDALL, PrimaryExaminer.

